Prior art speech-operated noise attenuation devices (SONAD) attenuate an audio signal in the absence of voice or speech signals to suppress the apparent level of background noise between syllables. These prior art circuits typically provide a degree of intersyllabic noise attenuation which is related to the prevailing noise level in the signal. When background noise levels vary, the noise suppression provided by the prior art SONAD circuits varies as well. In many audio signals, the level of background noise varies over a wide range so that the effectiveness of a prior art SONAD in removing intersyllabic noise will vary significantly. A SONAD that reduces background noise while accommodating changes in the background noise level is also known in the art.
Unfortunately, such prior art SONAD circuits produce a very unnatural sounding speech due to suppression of the background noise when operated under weak RF signal conditions. This is due to fast changes in attenuation, which produces a dramatic contrast between no attenuation and full attenuation. Such behavior is inevitable, since a single threshold point is used in order to achieve the desired degree of noise suppression. Other known SONAD circuits attempt to avoid the aforementioned problems by employing two attenuators in tandem, each having a different threshold point. Such a device is taught and suggested in U.S. Pat. No. 4,893,349 entitled "FM Communication System with Improved Response to Rayleigh-Faded Received Signals," issued Jan. 9, 1990, and assigned to the assignee of the present application. In accordance with the teachings of this prior art SONAD, the attenuation changes more rapidly as the input signal level is reduced. This improved attenuation characteristic tends to minimize speech distortions, since only the lower level speech sounds are significantly attenuated without affecting the higher level speech sounds. Furthermore, the use of separate threshold points more effectively reduces the noise between syllables to produce a better sounding speech than exhibited by prior art SONAD circuits.
Notwithstanding the production of more natural sounding speech accompanied by the use of the SONAD as taught in U.S. Pat. No. 4,893,349, this approach is still characterized by a fast change in attenuation which produces a contrast between no attenuation and full attenuation, in situations such as Rayleigh fading, and is therefore characterized by the production of speech which is still somewhat unnatural in its sound and makeup.
A need, therefore, exists to provide an improved audio noise reduction technique which is capable of reproducing natural sounding speech at both high RF signal strength levels and at low RF signal strength levels typically attributable to Rayleigh fading.